In an LTE-A (long-term evolution-advanced) system of Rel.10, in order to realize uplink communications, the synchronization of the uplink must be ensured. The process of realizing the synchronization of the uplink is as shown in FIG. 1, and comprises the steps of:
S1: transmitting a signal by a terminal in an uplink, the signal being an uplink random access preamble signal transmitted by the terminal in a random access procedure, or a data signal;
S2: judging by a base station according to the received signal, whether the uplink signal of the terminal needs to be adjusted, and further judging that a time advance operation is to be performed or a time lag operation is to be performed if the uplink signal needs to be adjusted. According to the result of judgment, the base station transmits a timing advance command on a downlink. In the time advance command, the base station notifies the terminal of the amount of time needed advance or lag. According to the type of the signal (a data signal or a random access preamble signal) transmitted by the terminal on the uplink, the time advance command transmitted by the base station is contained in different downlink messages. If a random access preamble signal is transmitted by the terminal in the uplink, the time advance command is contained in a random access response message. And if a data signal is transmitted by the terminal in the uplink, the time advance command is embodied as a control information element of a medium access control (MAC) layer.
S3: advancing or lagging the uplink transmission time by a corresponding amount by the terminal according to the indication of the time advance command after receiving the time advance command. And at the same time, a time advance timer needs to be maintained at the terminal. If the time advance timer is in operation, the terminal thinks that its uplink is in a synchronization state. And if the time advance timer expires, the terminal thinks that its uplink is out of synchronization, and needs to flush all the hybrid automatic repeat request (HARM) buffer, notify a radio resource control (RRC) layer to release a physical uplink control channel (PUCCH) and a sounding reference signal (SRS), and flush any configured downlink assignment and uplink grant. After the uplink is out of synchronization and if there is a need to transmit new uplink or downlink data, the terminal must execute a random access procedure, so as to obtain uplink synchronization again and restart the time advance timer.
Currently, there are several cases for the start of a time advance timer: a terminal starts or restarts the time advance timer when receiving an MAC element of a time advance command; starts or restarts the time advance timer when the time advance command is contained in a random access response message and if a random access preamble is not selected by an MAC layer of the terminal; starts the time advance timer when the time advance command is contained in a random access response message and a random access preamble is selected by an MAC layer of the terminal and if the time advance timer is not in operation, and stops the time advance timer when the contention in the random access procedure is not solved; and neglects the received time advance command in other cases where a time advance command is contained in a random access response message.
In a single-band system, for uplink carriers in the same band transmitted from the same terminal, when the positions of their receiving ends are identical, their synchronization states may be deemed as being consistent; that is, if an uplink of one carrier is synchronized by being adjusted, the uplinks of other carriers in the same band may also be synchronized only if the other carriers are adjusted in the same manner as that of the one carrier; therefore, in such a single-band system, uplink measurement may only be performed on one of the uplinks, and the other carriers are adjusted using the same measures according to the measurement result. In this way, the uplink synchronization of the whole band may be maintained, thereby only one time advance timer needing to be maintained at the terminal.
However, in a multi-band carrier aggregation system, relatively large differences exist in uplink transmission between different bands, and hence, there will exit difference in uplink synchronization adjustment for carriers of different bands. In such a case, different amounts of time adjustment need to be transmitted for the carriers of different bands, and it is possible that multiple time advance commands for multiple bands are transmitted simultaneously. And at the same time, multiple time advance timers may be maintained for multiple bands at the terminal, and also, one time advance timer may only be maintained.
Therefore, for a multi-band carrier aggregation system, how to maintain the one or more time advance timer(s) by a base station or a terminal has become a problem urgent to be solved.
FIG. 2 is a schematic diagram of multiple time advance timers (TATs) in a multi-band carrier aggregation system. As shown in FIG. 2, in the multi-band carrier aggregation system of this embodiment, a base station assigning three bands for a terminal is taken as an example, wherein, a primary cell (PCell) and a secondary cell (SCell) 1 are at a band 0, secondary cells 2 and 3 are at a band 1, and secondary cell 4 is at a band 2. In this embodiment, the terminal corresponding to the three bands via three different TATs is taken as an example.
As shown in FIG. 2, if the three TATs are maintained completely independently, when TAT0 is in operation and TAT1 expires, according to the uplink maintaining process, the terminal needs to flush the HARQ buffer in band 1 and release the dedicated resources to which the carriers in band 1 in the PUCCH correspond and the SRSs to which the carriers in band 1 correspond. Such operations relate only to the communications of the local band, and have no effect on the communications of other bands. Likewise, when TAT2 expires, the operations of the terminal have effect only on the communications of band 2. However, when TAT0 expires and TAT1 and TAT2 are in operation, the terminal judges that the uplink of the PCell is out of synchronization, and as PUCCHs are transmitted only in the uplink of the PCell, at this moment, due to the PUCCHs cannot be transmitted in the uplink, the communications of the whole terminal are affected, and cannot perform communications efficiently. In such a case, out-of-synchronization of one band leads to interruption of the communications of the whole terminal.
The case where TAT0 is in operation and TAT1 or TAT2 expires may be deemed as a normal situation, as in some cases, the base station may determine according to judgment of a traffic amount that some terminals do not need so many bands, and intentionally make uplinks of some bands be out of synchronization, so as to release resources for other terminals. However, the case where TAT1 and TAT2 are in operation and TAT0 expires should be avoided.
FIG. 3 is a schematic diagram of a single TAT in a multi-band carrier aggregation system. As shown in FIG. 3, in the multi-band carrier aggregation system of this embodiment, there is only one TAT at the terminal. In such a scenario, if the TAT is made to be reset by the time advance command to which a certain band corresponds, the terminal cannot learn the uplink synchronization situation of all the bands. For example, if the time advance command to which band 0 corresponds can only reset the TAT, the terminal will not learn whether band 1 is in synchronization. If the TAT is made to be reset by the time advance command to which any band corresponds, following situation is possible to occur: in the time period set in the timer, multiple time advance commands are received at a certain band, and at the same time, no time advance command is received at certain another band, while the timer is still in operation. Based on the TAT principle, in the time period set in the timer, the base station should be able to reasonably set the numerical values in a time advance command and transmission timing according to the uplink synchronization situation, and keeps the timer in operation. And if the terminal receives no time advance command in the set time period, it is deemed as out-of-synchronization of uplink. In such a case, the uplink state obtained by the terminal from the TAT is wrong. For example, the numerical value set by TAT0 in FIG. 3 is 500 ms. Within 500 ms, the terminal receives twice the time advance command corresponding to band 1, while receiving no time advance command corresponding to band 0. At this moment, the TAT is in operation, but the uplink of band 0 is actually possible to be out of synchronization.
A document advantageous to the understanding of the present invention and the conventional technologies is listed below, which is incorporated herein by reference, as it is fully described herein.    [Non-patent document 1]: 3GPP TS 36.321 C10.1.0(2011-3) Medium Access Control (MAC) protocol specification.
It should be noted that the above description of the background art is merely provided for flush and complete explanation of the present invention and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background art of the present invention.